A paper by Hughes et al entitled "Identification of Two Related Pentapeptides from the Brain with Potent Opiate Agonist Activity", Nature, 258, 577-579 (1975), reports the purification and identification of an endogenous substance from the extracts of pig brains termed "enkephalin" which acts as an agonist at opiate receptor sites. Hughes et al also report the enkephalin is comprised of two related pentapeptides (a) H-Tyr-Gly-Gly-Phe-Met-OH and (b) H-Tyr-Gly-Gly-Phe-Leu-OH (herein enkephalin sequence one and sequence two, respectively), in a ratio of approximately 3:1. Enkephalin is reported to be more potent than morphine, sequence one being more potent than sequence two. Both sequences have been synthesized and the synthetic pentapeptides illustrate similar opiate agonist effects to the naturally occuring substance.